This invention relates to hydraulic braking systems for vehicles of the kind in which a pedal-operated booster-assisted hydraulic master cylinder assembly is adapted to apply a vehicle brake by the supply of hydraulic fluid under pressure to a brake actuator for operating the brake.
In braking systems, particularly for vehicles of the xe2x80x9coff-highway typexe2x80x9d, such as tractors and earth moving equipment, the trend is to adopt large running clearances between the braking members of the brake, for example between friction members and rotatable discs, in order that friction loses arising from parasitic drag are reduced or substantially eliminated.
We are aware of GB-A-2179715 which forms the pre-characterising portion of claims 1 and 8.
According to our invention in an hydraulic braking system of the kind set forth for vehicles, a proportional valve is arranged to provide a quick-fill function for a wheel brake actuator in response to a proportional pilot signal from the master cylinder assembly, and the booster includes first and second pistons which are relatively moveable to close a port leading to a reservoir and to open a proportional pressure port through which the pilot signal comprising hydraulic pressure generated in the space between the pistons by relative movement therebetween is directed to and opens the proportional valve.
The quick-fill function compensates for additional fluid volume requirements, and ensures that the pedal is immediately responsive with such additional volume requirements.
The proportional valve may be situated in any convenient location, for example in close proximity to the brake actuator.
In one construction, in a normal operative mode, the booster is adapted to meter input pressure to provide a booster signal, with the metered side of the booster being connected into and at a pressure, substantially equal to the pilot signal of the proportional valve.
As the pilot signal is applied to the proportional valve, the proportional valve will open, allowing hydraulic fluid to flow from available vehicle hydraulics into the brake actuator.
The xe2x80x9cquick-fillxe2x80x9d function works until a pre-set pressure is reached, directly proportional to the pilot signal provided by the booster which is also directly proportional to the effort applied by the pedal.
Preferably the setting of the proportional valve is such that all clearances are taken up by utilising available vehicle hydraulics and thereafter the proportional valve is adapted to close allowing all further braking to be applied directly from the master cylinder.
The invention allows rationalisation of boosters into fewer sizes to meet customer requirements. By tuning the proportional valve or pre-setting the setting of the proportional valve, the clearances of the brake may be taken up accordingly, and higher pressure brake stiffnesses may be applied through one size of master cylinder for all applications.
The invention is particularly applicable to a dual braking system in which brakes on wheels on opposite sides of the vehicle may be operated simultaneously for braking, or independently to assist steering. When operated simultaneously pressure spaces in the two master cylinder assemblies are interconnected to compensate for differential wear of linings of the brakes.